Add support for context manager dependencies

[cofob]

First Check

• I added a very descriptive title here.
• I used the GitHub search to find a similar question and didn't find it.
• I searched the FastAPI documentation, with the integrated search.
• I already searched in Google "How to X in FastAPI" and didn't find any information.
• I already read and followed all the tutorial in the docs and didn't find an answer.
• I already checked if it is not related to FastAPI but to Pydantic.
• I already checked if it is not related to FastAPI but to Swagger UI.
• I already checked if it is not related to FastAPI but to ReDoc.

Example Code

from typing import Annotated, Generator
from fastapi import FastAPI, Depends
from contextlib import contextmanager

app = FastAPI()


def generator_dependency() -> Generator[str, None, None]:
    # setup_work_here()
    try:
        yield "some useful data"
    finally:
        pass
        # shutdown_work_here()


@contextmanager
def contextmanager_dependency() -> Generator[str, None, None]:
    # setup_work_here()
    try:
        yield "some useful data"
    finally:
        pass
        # shutdown_work_here()


@app.get("/generator")
def generator_endpoint(dep: Annotated[str, Depends(generator_dependency)]) -> str:
    return dep


@app.get("/contextmanager")
def contextmanager_endpoint(dep: Annotated[str, Depends(contextmanager_dependency)]) -> str:
    return dep

Description

I expect /contextmanager to work the same as /generator, but it doesn't. FastAPI does not use contextmanager_dependency as a context manager, and instead of executing it and substituting the result, it just substitutes the initialized contextlib._GeneratorContextManager class into dep.

I suggest adding support for contextmanager and asynccontextmanager to the DI system, as they are more convenient to use outside of the DI system.

Operating System

Linux

Operating System Details

No response

FastAPI Version

0.109.2

Pydantic Version

2.6.1

Python Version

Python 3.12.1

Additional Context

Exception traceback at /contextmanager:

INFO:     127.0.0.1:59372 - "GET /contextmanager HTTP/1.1" 500 Internal Server Error
ERROR:    Exception in ASGI application
Traceback (most recent call last):
  File "/REDACTED/lib/python3.12/site-packages/uvicorn/protocols/http/httptools_impl.py", line 419, in run_asgi
    result = await app(  # type: ignore[func-returns-value]
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/REDACTED/lib/python3.12/site-packages/uvicorn/middleware/proxy_headers.py", line 84, in __call__
    return await self.app(scope, receive, send)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/REDACTED/lib/python3.12/site-packages/fastapi/applications.py", line 1054, in __call__
    await super().__call__(scope, receive, send)
  File "/REDACTED/lib/python3.12/site-packages/starlette/applications.py", line 123, in __call__
    await self.middleware_stack(scope, receive, send)
  File "/REDACTED/lib/python3.12/site-packages/starlette/middleware/errors.py", line 186, in __call__
    raise exc
  File "/REDACTED/lib/python3.12/site-packages/starlette/middleware/errors.py", line 164, in __call__
    await self.app(scope, receive, _send)
  File "/REDACTED/lib/python3.12/site-packages/starlette/middleware/exceptions.py", line 62, in __call__
    await wrap_app_handling_exceptions(self.app, conn)(scope, receive, send)
  File "/REDACTED/lib/python3.12/site-packages/starlette/_exception_handler.py", line 64, in wrapped_app
    raise exc
  File "/REDACTED/lib/python3.12/site-packages/starlette/_exception_handler.py", line 53, in wrapped_app
    await app(scope, receive, sender)
  File "/REDACTED/lib/python3.12/site-packages/starlette/routing.py", line 758, in __call__
    await self.middleware_stack(scope, receive, send)
  File "/REDACTED/lib/python3.12/site-packages/starlette/routing.py", line 778, in app
    await route.handle(scope, receive, send)
  File "/REDACTED/lib/python3.12/site-packages/starlette/routing.py", line 299, in handle
    await self.app(scope, receive, send)
  File "/REDACTED/lib/python3.12/site-packages/starlette/routing.py", line 79, in app
    await wrap_app_handling_exceptions(app, request)(scope, receive, send)
  File "/REDACTED/lib/python3.12/site-packages/starlette/_exception_handler.py", line 64, in wrapped_app
    raise exc
  File "/REDACTED/lib/python3.12/site-packages/starlette/_exception_handler.py", line 53, in wrapped_app
    await app(scope, receive, sender)
  File "/REDACTED/lib/python3.12/site-packages/starlette/routing.py", line 74, in app
    response = await func(request)
               ^^^^^^^^^^^^^^^^^^^
  File "/REDACTED/lib/python3.12/site-packages/fastapi/routing.py", line 315, in app
    content = await serialize_response(
              ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/REDACTED/lib/python3.12/site-packages/fastapi/routing.py", line 155, in serialize_response
    raise ResponseValidationError(
fastapi.exceptions.ResponseValidationError: 1 validation errors:
  {'type': 'string_type', 'loc': ('response',), 'msg': 'Input should be a valid string', 'input': <contextlib._GeneratorContextManager object at 0x7f85bcb6e6f0>, 'url': 'https://errors.pydantic.dev/2.6/v/string_type'}

[cofob]

There is no solution for this at this time. It's sad that the maintainers are not following the project and responding to issues. I would have been ready to implement this, except there has been absolutely no response. Closing.

[shawnz]

Please leave this issue open as it's still a concern for me and surely many other users!

[Danipulok]

@tiangolo good day!
Was looking for context issues and found this.
It seems @cofob would be ready to maybe do a PR if the idea gets approved by you.
In my team we have some workaround for this, but it's not convenient at all so I'm sure 100% it's popular and on high demand.
Any possibility you could approve the idea?